The present invention relates to drive tractors which are widely used in printers and other devices for handling webs such as continuous perforated paper, individual sheet paper, tape and the like, fabricated of paper, foil, laminates and other sheet-like material, all collectively referred to hereinafter as "web material".
Generally such devices employ a belt which will engage the web material either by pins or projections on the upper surface of the belt extending into perforations in the web material, or by friction or "clamping" of the web material between the upper surface of the belt and a cover or guide thereabove. Such devices are in widespread use for the applications indicated above and are particularly burgeoning in usage for paper handling in printers for computers, word processing and duplicating apparatus.
In Seitz U.S. Pat. No. 4,130,23 granted Dec. 19, 1978; U.S. Pat. No. 4,194,660 granted Mar. 25, 1980; and U.S. Pat. No. 4,315,585 granted Feb. 16, 1982, there are illustrated and described tractors of this type employing a one-piece integrally molded continuous synthetic resin belt travelling in a continous path about a drive sprocket or pulley with which it is drivingly engaged and a cooperating arcuate surface, which is generally a second pulley or sprocket which idles. The paper or other web is driven by the belt as it passes between it and an overlying cover. The belt desirably has a diameter sufficient to provide a linear length between the pulleys greater than the rectilinear length therebetween so that the excess length will absorb stresses on the belt during operation. This type of one-piece molded belt has an inherent tendency to arch outwardly between the drive pulley and cooperating arcuate surface because of its memory resulting from its being molded as a circle. As a result, the web is biased upwardly against the cover, and the web is fully seated on the drive pins on the upper surface of the belt.
Particularly in tractors which rapidly intermittently move the web, or which rapidly move it backwards as well as forwards, both common as in printers, it is desirable to provide some slack in the belt between its guide surfaces to absorb some of the forces resulting from the rapid reversal or termination of drive motion. However, it is also desirable to avoid downward deflection of the belt between the guide surfaces to ensure that the belt holds the web tightly against the cover and thereby firmly seated on the drive pins. Moreover, it is desirable to avoid having the lower surface of the web bear upon the upper surface of the tractor chassis.
It is an object of the present invention to provide a novel drive tractor for web material which provides resilient tensioning of a drive belt between its guide surface to bias the belt against an overlying guide surface.
It is also an object to provide such a drive tractor which may be fabricated readily and relatively economically and which will enjoy long life in operation.
Another object is to provide such a tractor wherein the web path remains substantially rectilinear despite rapid changes in direction of belt travel and wherein there are avoided variations in line spacing in the web.